In most zoom lenses the front group of lens elements is moved along the optical axis to achieve focusing from infinity to moderately close distances, typically having a magnification of 1:10 or less. To enable these lenses to focus still closer, in a macro or close focusing mode, the lens is adjusted by suitable repositioning of one or more of the zooming groups (the variator group and compensator group) in a manner different from the movements in the normal zoom range.
An example of such optics is disclosed in U.S. Pat. No. 3,817,600 where the zooming elements are moved in relationship different from the normal zooming movements to achieve close focusing below two meters up to a magnification of 1:2.2 as close as forty-five centimeters from the front element. In this lens, the variator and compensator are moved in fixed relation. In other lenses, the variator may be moved independent of movement of the compensator to achieve closer focusing than the normal zoom range.
A lens of this type disclosed in U.S. Pat. No. 3,817,600 has proven to be an excellent optic and has been marketed as a Vivitar Series I Lens of 70-210 mm equivalent focal length. In the close focusing mode, it is possible to achieve magnifications of 1:2.2. However, this lens does require repositioning of the zooming groups for different movements for close focusing.
Zoom lenses generally do not have an optically strong front group. Conventional zoom lenses are characterized by a weak power (long focal length) first group, which has been believed to be the best way to minimize aberration change due to focusing.
The present invention provides a compact mechanically compensated zoom lens having the capability of focusing continuously from infinity down to the macro or close focusing mode by small movements of a front focusing group while maintaining compactness of the lens.